Fatigue crack growth resistance of the austenitic stainless steel Alloy 709 at elevated temperatures

Fatigue crack growth resistance of an austenitic stainless steel Alloy 709 has been evaluated at temperatures of 550, 650 and 750 °C in air and vacuum. Tests were conducted at a frequency of 0.25 Hz and a stress ratio of 0.1. The linear elastic stress intensity factor range (ΔK) has been used to characterise fatigue crack growth resistance. A modest detrimental effect of air at elevated temperatures on fatigue crack growth is identified and discussed. Striated transgranular fatigue is found to be the failure mechanism for all test conditions. The formation of striations and the interaction of crack growth with slip traces are further investigated using in-situ testing (within a scanning electron microscope), together with transmission electron microscopy carried out on samples extracted by focused ion-beam milling perpendicular to fracture surfaces. Finally, an analytical model is proposed to predict fatigue crack growth in air for Alloy 709.